A Comparison of Spreading Angles of Turbulent Wedges in Velocity and Thermal Boundary Layers

2003 ◽  
Vol 125 (2) ◽  
pp. 267-274 ◽  
Author(s):  
S. Zhong ◽  
T. P. Chong ◽  
H. P. Hodson
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Liquid Crystals ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Temperature Sensitive ◽  
Shear Stresses ◽  
Zone Length ◽  
The Difference ◽  
Thermal Boundary Layers

Turbulent wedges induced by a three-dimensional surface roughness placed in a laminar boundary layer over a flat plate were visualized for the first time using both shear-sensitive and temperature-sensitive liquid crystals. The experiments were carried out at zero pressure gradient and two different levels of favorable pressure gradients. The purpose of this investigation was to examine the spreading angles of turbulent wedges indicated by their associated surface shear stresses and heat transfer characteristics and hence obtain further insight about the difference in the behavior of transitional momentum and thermal boundary layers when a streamwise pressure gradient exists. It was found that under a zero pressure gradient the spreading angles indicated by the two types of liquid crystals are the same, but the difference increases as the level of favorable pressure gradient increases with the angle indicated by temperature-sensitive liquid crystals being smaller. The results from the present study suggest that the spanwise growth of a turbulent region is smaller in a thermal boundary layer than in its momentum counterpart and this seems to be responsible for the inconsistency in transition zone length indicated by the distribution of heat transfer rate and boundary layer shape factor reported in the literature. This finding would have an important implication to the transition modeling of thermal boundary layers over gas turbine blades.

Visualisation of Turbulent Wedges Under Favourable Pressure Gradients Using Both Shear-Sensitive and Temperature-Sensitive Liquid Crystals

2002 ◽  
Author(s):  
S. Zhong ◽  
T. P. Chong ◽  
H. P. Hodson
Keyword(s):  
Liquid Crystals ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Turbine Blades ◽  
Temperature Sensitive ◽  
Shear Stresses ◽  
Pressure Gradients ◽  
Surface Shear ◽  
The Difference ◽  
Thermal Boundary Layers

Turbulent wedges induced by a 3D surface roughness placed in a laminar boundary layer over a flat plate were visualised for the first time using both shear-sensitive and temperature-sensitive liquid crystals. The experiments were carried out at three different levels of favourable pressure gradients. The purpose of this investigation was to examine the spreading angles of the turbulent wedges indicated by their associated surface shear stresses and heat transfer characteristics and hence obtain further insight about the difference in the behaviour of transitional momentum and thermal boundary layers when a streamwise pressure gradient exists. It was shown that under a zero pressure gradient the spreading angles indicated by the two types of liquid crystals are the same, but the difference increases as the level of favourable pressure gradient increases. The result from the present study could have an important implication to the transition modelling of thermal boundary layers over gas turbine blades.

On the Spreading Angle of Turbulent Spots in Non-Isothermal Boundary Layers With Favourable Pressure Gradients

2002 ◽  
Author(s):  
S. Zhong ◽  
T. P. Chong ◽  
H. P. Hodson
Keyword(s):  
Liquid Crystals ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Turbine Blades ◽  
Temperature Sensitive ◽  
Shear Stresses ◽  
Pressure Gradients ◽  
Turbulent Spots ◽  
The Difference ◽  
Thermal Boundary Layers

Turbulent spots created artificially in a flat plate boundary layer were visualized using both shear-sensitive and temperature-sensitive liquid crystals for the first time. The experiments were carried out at three different levels of favourable pressure gradients. The purpose of this investigation was to examine the spreading angles of the turbulent spots indicated by their associated surface shear stresses and heat transfer characteristics and hence obtain further insight about the difference in the behaviour of transitional momentum and thermal boundary layers when a streamwise pressure gradient is present. It was shown that under a zero pressure gradient the spreading angles indicated by the two types of liquid crystals are the same, but the difference increases as the level of favourable pressure gradient increases. The result from the present study could have an important implication for the transition modelling of thermal boundary layers over gas turbine blades. Further investigations are to be carried out.

On the Momentum and Thermal Structures of Turbulent Spots in a Favorable Pressure Gradient

2005 ◽  
Vol 128 (4) ◽  
pp. 689-698 ◽  
Author(s):  
T. P. Chong ◽  
S. Zhong
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Velocity Component ◽  
Thermal Boundary Layer ◽  
Temperature Fluctuations ◽  
Turbulent Spots ◽  
Zone Length ◽  
Favorable Pressure Gradient ◽  
The Difference

This paper represents the results from an experimental investigation of the flow physics behind the difference in the transition zone length indicated by the momentum boundary layer and thermal boundary layer parameters observed on the suction surfaces of gas turbine blades. The experiments were carried out on turbulent spots created artificially in an otherwise laminar boundary layer developing over a heated flat plate in a zero pressure gradient and a favorable pressure gradient. A specially designed miniature triple wire probe was used to measure the streamwise velocity component U, transverse velocity component V and temperature T simultaneously during the passage of the spots. In this paper, the general characteristics of the ensemble-averaged velocity and temperature perturbations, rms fluctuations, and the second moment turbulent quantities are discussed and the influence of favorable pressure gradient on these parameters is examined. When a favorable pressure gradient is present, unlike in the velocity boundary layer where significant velocity fluctuations and Reynolds shear stress occur both on the plane of symmetry and the spanwise periphery, high temperature fluctuations (and turbulent heat fluxes) are confined in the plane of symmetry. The difference in the levels of velocity/temperature fluctuations at these two locations gives an indication of the effectiveness of momentum/heat transfer across the span of the spots. The results of this study indicate that the heat transfer within a spot is inhibited more than that of the momentum transfer at the presence of a favorable pressure gradient. This phenomenon is expected to slow down the development of a transitional thermal boundary layer, leading to a longer transitional zone length indicated by the heat transfer parameters as reported in the literature.

On the Momentum and Thermal Structures of Turbulent Spots in a Favourable Pressure Gradient

2005 ◽  
Author(s):  
T. P. Chong ◽  
S. Zhong
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Thermal Boundary Layer ◽  
Turbine Blades ◽  
Temperature Fluctuations ◽  
Turbulent Heat ◽  
Turbulent Spots ◽  
Zone Length ◽  
The Difference

This paper represents the results from an experimental investigation of the flow physics behind the difference in the transition zone length indicated by the momentum boundary layer and thermal boundary layer parameters observed on the suction surfaces of gas turbine blades. The experiments were carried out on turbulent spots created artificially in an otherwise laminar boundary layer developing over a heated flat plate in a zero pressure gradient and a favourable pressure gradient. A specially designed miniature triple wire probe was used to measure the streamwise velocity U, transverse velocity component V and temperature T simultaneously during the passage of the spots. In this paper, the general characteristics of the ensemble-averaged velocity and temperature perturbations, rms fluctuations and the second moment turbulent quantities are discussed and the influence of favourable pressure gradient on these parameters is examined. When a favourable pressure gradient is present, unlike in the velocity boundary layer where significant velocity fluctuations (or Reynolds shear stress) occur both on the plane of symmetry and the spanwise periphery, high temperature fluctuations (or turbulent heat fluxes) are confined in the plane of symmetry. The difference in the levels of velocity/temperature fluctuations at these two locations gives an indication of the effectiveness of momentum/heat transfer across the span of the spots. The results of this study show that the heat transfer within a spot is inhibited more than that of the momentum transfer at the presence of a favourable pressure gradient. This phenomenon is expected to slow down the spanwise growth of turbulent spots in the transitional thermal boundary layer, leading to a longer transitional zone length indicated by the heat transfer parameters as reported in the literature.

Characteristics of Hydro-Dynamically and Thermally Developing Liquid Flow With Micro-Encapsulated Phase Change Material

2009 ◽  
Author(s):  
Rami Sabbah ◽  
Jamal Yagoobi ◽  
Said Al Hallaj
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Phase Change ◽  
Phase Change Material ◽  
Boundary Layers ◽  
Liquid Flow ◽  
Thermal Boundary Layer ◽  
Encapsulated Phase Change Material ◽  
Change Material ◽  
Thermal Boundary Layers

This numerical investigation explores the hydrodynamic and thermal boundary layers characteristics of a liquid flow with Micro-Encapsulated Phase Change Material (MEPCM). Unlike pure liquids, the heat transfer characteristics of MEPCM slurry can not be simply presented in terms of corresponding dimensionless controlling parameters such as Peclet number. In the presence of phase change particles, the controlling parameters’ values change significantly along the tube length due to the phase change. As a result, the hydrodynamic and thermal boundary layers are significantly affected by the changing parameters. The numerical results reveal that the growth of the thermal boundary layer for MEPCM slurries is different than for pure liquids. The presence of MEPCM in the working fluid slows the growth of the thermal boundary layer and extends the thermal entry length. The local heat transfer coefficient strongly depends on the location of the melting zone interface.

Protuberances in a Turbulent Thermal Boundary Layer

2011 ◽  
Author(s):  
Steven R. Mart ◽  
Stephen T. McClain
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Flux ◽  
Boundary Layers ◽  
Thermal Boundary Layer ◽  
Leading Edge ◽  
Heated Plate ◽  
Fluid Temperature ◽  
Constant Flux ◽  
Thermal Boundary Layers

Recent efforts to evaluate the effects of isolated protuberances within velocity and thermal boundary layers have been performed using transient heat transfer approaches. While these approaches provide accurate and highly resolved measurements of surface flux, measuring the state of the thermal boundary-layer during transient tests with high spatial resolution presents several challenges. As such, the heat transfer enhancement evaluated during transient tests are presently correlated to a Reynolds number based either on the distance from the leading edge or on the momentum thickness. Heat flux and temperature variations along the surface of a turbine blade may cause significant differences between the shapes and sizes of the velocity and thermal boundary layer profiles. Therefore, correlations are needed which relate the states of both the velocity and thermal boundary layers to protuberance and roughness distribution heat transfer. In this study, a series of three experiments are performed for various freestream velocities to investigate the local temperature details of protuberances interacting with thermal boundary layers. The experimental measurements are performed using isolated protuberances of varying thermal conductivity on a steadily-heated, constant flux flat plate. In the first experiment, detailed surface temperature maps are recorded using infrared thermography. In the second experiment, the unperturbed velocity profile over the plate without heating is measured using hot-wire anemometry. Finally, the thermal boundary layer over the steadily heated plate is measured using a thermocouple probe. Because of the constant flux experimental configuration, the protuberances provide negligible heat flux augmentation. Consequently, the variation in protuberance temperature is investigated using the velocity boundary layer parameters, the thermal boundary layer parameters, and the local fluid temperature at the protuberance apices. A comparison of results using plastic and steel protuberances illuminates the importance of the shape of the thermal and velocity boundary layers in determining the minimum protuberance temperatures.

scholarly journals Heat Transfer and Fluid Dynamics Measurements in Accelerated Rough-Wall Boundary Layer

1993 ◽  
Author(s):  
Walid Chakroun ◽  
Robert P. Taylor
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Rough Wall ◽  
Rough Boundary ◽  
Flow Conditions ◽  
Wall Boundary Layer

The combined effects of freestream acceleration and surface roughness on heat transfer and fluid dynamics in the turbulent boundary layer were investigated experimentally. The experiments included a variety of flow conditions ranging from aerodynamically-smooth through transitionally-rough to fully-rough boundary layers with accelerations ranging from moderate to modestly strong. Two well-defined rough surfaces composed of 1.27 mm diameter hemispheres spaced 2 and 4 diameters apart, respectively, in staggered arrays on otherwise smooth surfaces were used as the test surfaces. The first 1.5 m of the test section had zero-pressure gradient followed by a 0.4 m accelerated region with the remaining 0.4 m adjusted to zero-pressure gradient. The Stanton number for the rough-wall experiments decreased or increased for accelerated rough-wall cases compared to zero-pressure gradient cases depending on flow conditions. For fully-rough boundary layers, Stanton numbers increased with acceleration compared to zero-pressure gradient at the same x-position. For aerodynamically-smooth and transitionally-rough boundary-layer flows, the effect of acceleration was not similar to that of fully-rough flows and was highly dependent upon the flow conditions. The acceleration caused a decrease in the relative turbulence level over the rough surface. The profiles of u′2¯ for the accelerated runs were lower than those of zero-pressure gradient cases, and a substantial decrease in the Reynolds shear stress (−u′v′¯) component was observed when acceleration was applied.

Protuberances in a Turbulent Thermal Boundary Layer

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Steven R. Mart ◽  
Stephen T. McClain
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Flux ◽  
Boundary Layers ◽  
Thermal Boundary Layer ◽  
Leading Edge ◽  
Heated Plate ◽  
Fluid Temperature ◽  
Constant Flux ◽  
Thermal Boundary Layers

Recent efforts to evaluate the effects of isolated protuberances within velocity and thermal boundary layers have been performed using transient heat transfer approaches. While these approaches provide accurate and highly resolved measurements of surface flux, measuring the state of the thermal boundary layer during transient tests with high spatial resolution presents several challenges. As such, the heat transfer enhancement evaluated during transient tests is presently correlated to a Reynolds number based either on the distance from the leading edge or on the momentum thickness. Heat flux and temperature variations along the surface of a turbine blade may cause significant differences between the shapes and sizes of the velocity and thermal boundary layer profiles. Therefore, correlations are needed which relate the states of both the velocity and thermal boundary layers to protuberance and roughness distribution heat transfer. In this study, a series of three experiments are performed for various freestream velocities to investigate the local temperature details of protuberances interacting with thermal boundary layers. The experimental measurements are performed using isolated protuberances of varying thermal conductivity on a steadily heated, constant flux flat plate. In the first experiment, detailed surface temperature maps are recorded using infrared thermography. In the second experiment, the unperturbed velocity profile over the plate without heating is measured using hot-wire anemometry. Finally, the thermal boundary layer over the steadily heated plate is measured using a thermocouple probe. Because of the constant flux experimental configuration, the protuberances provide negligible heat flux augmentation. Consequently, the variation in protuberance temperature is investigated using the velocity boundary layer parameters, the thermal boundary layer parameters, and the local fluid temperature at the protuberance apices. A comparison of results using plastic and steel protuberances illuminates the importance of the shape of the thermal and velocity boundary layers in determining the minimum protuberance temperatures.

Laminar boundary layers behind detonation waves

1983 ◽  
Vol 387 (1793) ◽  
pp. 331-349 ◽  
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Boundary Layers ◽  
Flow Analysis ◽  
Time Dependent ◽  
Detonation Waves ◽  
Planar Geometry ◽  
Laminar Boundary Layers ◽  
Spherical Detonation ◽  
Layer Flow

New solutions are presented for non-stationary boundary layers induced by planar, cylindrical and spherical Chapman-Jouguet (C-J) detonation waves. The numerical results show that the Prandtl number ( Pr ) has a very significant influence on the boundary-layer-flow structure. A comparison with available time-dependent heat-transfer measurements in a planar geometry in a 2H 2 + O 2 mixture shows much better agreement with the present analysis than has been obtained previously by others. This lends confidence to the new results on boundary layers induced by cylindrical and spherical detonation waves. Only the spherical-flow analysis is given here in detail for brevity.

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